You can do a fast download of all the unofficial LDraw parts from here.

Generally, we recommend you grab unofficial parts one-by-one, and use them for only for a specific project.
If you are using MLCAD, you can import the unofficial part(s) as submodels into your main model files.
That way, if changes are made to the unofficial parts before they are officially released, your model won't be
adversely affected.

But, if you're impatient, sure you want all of the unofficial parts, and are willing to take your chances,
you can download all of the unofficial parts at once.

You will need to clean up at a later time.
The *best* (that is, the safest) way to install these files is to unzip them to a temporary directory,
then move them to your LDraw\models directory.
Don't dump them into your LDraw\parts directory, it would be very hard to sort them out of there later.

When you unzip the files, be sure to tell your ZIP tool to use the folder names,
so all the files will end up in their proper subdirectory.